U.S. Pat. No. 3,791,466 "Low Parasitic Capture Fuel Assembly Structure" describes many methods of attaching nuclear fuel assembly grids to the assembly's guide thimble or guide tube structure.
In the past it has been common to weld Inconel lower grids to the Stainless Steel lower end fittings by use of the Inconel lower grid's perimeter strip. Other Zircaloy grids are attached to the guide tubes by welding of their strips directly to the Zircaloy guide tubes.
Today, certain European countries require that fuel assemblies be reconstitutable from the bottom of the fuel assembly. This in turn requires that the attachment of the lower grid to the bottom nozzle be removable. Thus making welding of the lower grid to the lower end fitting an unattractive method of attachment because welds must first be broken before the lower end fitting can be removed for assembly reconstitution. To eliminate this condition, it is deemed desirable to have attachments which can secure the lower grid to the lower end fitting while allowing it to be removed and reattached.